This invention relates to a method of producing semiconductor devices, particularly, to an improvement in the method of forming an interconnection electrode for a semiconductor device.
For forming an interconnection electrode of a semiconductor device, it is popular to remove selectively a conductive material layer by photolithography so as to form an interconnection electrode of a desired pattern. However, this method gives rise to various drawbacks including the following:
1. Where the bonding strength between the conductive material layer and photoresist is not sufficiently high, it is difficult to form an interconnection electrode of a high accuracy. PA0 2. Where the conductive material layer is difficult to be etched, it is difficult to form an interconnection electrode itself. PA0 3. Where it is intended to form an interconnection electrode consisting of a plurality of different metal layers, it is necessary to employ a plurality of etching steps using different etchants. PA0 1. Since the lift-off method utilizes discontinuity of a conductive material layer at the wall defining an opening of a spacer, it is necessary to use a relatively thick resin layer as the spacer, rendering it difficult to form a fine interconnection electrode. PA0 2. Since the spacer resin layer is formed by spin coating, the resin layer becomes progressively thicker toward the periphery thereof, resulting in an unnecessary interconnection between the electrode and the resin layer remaining around the insulating layer after the lift-off step. Further, fine resin residue remains on the insulating layer after the lift-off step. It should be noted that the resin residue is decomposed in the annealing step after formation of the interconnection electrode, causing contamination of the semiconductor device. PA0 3. In general, the substrate is kept heated in the step of forming the conductive material layer so as to enable the conductive material to cover the step portion between the substrate and the insulating layer without fail. But, the spacer resin is melted or decomposed by the heat and, thus, rendered incapable of performing the spacer function. If the substrate is not heated, the interconnection electrode, i.e., conductive material layer, fails to cover sufficiently the step portion between the substrate and the insulating layer. PA0 4. The impurities unavoidably contained in the resin layer cause contamination of the semiconductor device in the step of forming the conductive material layer or in the heat treatment step after formation of the interconnection electrode, leading to deterioration of the element characteristics. If cleaning treatment using a mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 is applied to the resin layer for removing the impurities, the resin layer itself is dissolved out, failing to perform the spacer function.
It is also proposed to employ a lift-off method for forming an interconnection electrode. In this method, a spacer consisting of photoresist or polyimide resin is formed on a silicon oxide insulating layer formed on a semiconductor substrate. The spacer is provided with an opening for forming an interconnection electrode and is used as a mask for selectively etching away the insulating layer so as to form an opening for contact. Then, a conductive material layer is formed on the entire surface. What should be noted is that the conductive material layer is rendered discontinuous at the wall defining the opening of the spacer. Thus, upon removal of the spacer, the conductive material layer formed on the spacer is removed together with the spacer so as to form an interconnection electrode. However, the lift-off method utilizing resin such as photoresist as a spacer gives rise to various drawbacks including the following: